Hollow fiber membrane contactor scrubber/stripper for cabin carbon dioxide and humidity control

ABSTRACT

An environmental control system includes an air conditioning subsystem and a contaminant removal subsystem downstream of the environment to be conditioned. The contaminant removal subsystem includes: a first gas-liquid contactor-separator; a second gas-liquid contactor-separator; and a dehumidifier disposed either upstream of the first gas-liquid contactor-separator or downstream of the second gas-liquid contactor-separator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.62/463,921, filed Feb. 27, 2017.

BACKGROUND OF THE INVENTION

The present invention generally relates to contaminant removal and, moreparticularly, to apparatus and methods of contaminant removal employinggas-liquid contact and separation.

It is of great interest to control and limit the concentration of carbondioxide (CO₂) in occupied spaces, including homes, buildings,transportation vehicles, aircraft and spacecraft. It is particularlyimportant to control CO₂ concentrations in enclosed vehicles likeaircraft or spacecraft. In aircraft, fresh air enters the occupied spaceas bleed air from the engine, and results in increased fuel consumption.Decreasing the bleed air flow would improve fuel efficiency, but wouldrequire a technology to remove CO₂ from the air. The Federal AviationAdministration (FAA) of the United States limits the acceptableconcentration of CO₂ to 5000 ppm, while aircraft typically have1500-2300 ppm. In spacecraft, no fresh air is available, and the cabinair must be preserved in a healthful condition.

Crews of the International Space Station (ISS), with elevated CO₂ levelsjust under 4 mmHg (5300 ppm) have reported symptoms such as earlyfatigue onset, impaired function and decision-making, and headaches. LawJ, Alexander D (2016). CO2 on the International Space Station: AnOperations Update. Annual AsMA Meeting; 24-28 Apr. 2016; Atlantic City,N.J., USA;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019624.pdf.Long duration, deep space missions lengthen crew exposure to theseconditions. NASA is requiring future spacecraft to maintain CO₂ partialpressures in the vessel atmosphere below 2 torr (2600 ppm) to preservecrew health, and maintain alertness and comfort. Therefore, moreadvanced CO₂ removal systems are required for next generation deep spacevehicles in order to maintain a much lower CO₂ partial pressure. Inaddition, deep space vehicles are required to have a lower size, weight,power, and thermal load, and use fewer consumables, while fixingexisting safety problems that are apparent in current systems. Themaintenance interval of current systems (three to six months) is alsorequired to jump to three years.

CO₂ recovery and recycling is a critical component of the airrevitalization system for long duration missions. Presently on ISS, thecarbon cycle, or carbon loop, is not closed and CO₂ is either discardedto space or processed through a Sabatier reactor to recover water;methane produced by the Sabatier reactor is discarded to space. Longerduration missions will require a more closed carbon loop to minimizecarrying disposable resources in the vessel—such as water, hydrogen,oxygen, etc.—that might otherwise be replenished from recycling CO₂. Inother words, any discarded carbon dioxide increases in the amount ofoxygen or water required to be brought with the mission.

For applications in spacecraft or aircraft, the size and weight of theoverall system must be minimized. Great emphasis must be placed onminimizing the size, weight and number of scrubber or stripper modules.It is well known that stripper modules can be made to be more efficientif a sweep gas is used to flush the permeate out of the module, and thatthis can minimize the size, weight and number of such modules. But thesource of this sweep gas is also important, since if it is foreign tothe process then a supply of this gas must be provided.

Such a closed-loop CO₂ recovery system should be capable of throttlingits process when process demand is lower in order to reduce energyconsumption. For missions to Mars, some plans include landing on theplanet and remaining there for eighteen months, during which the MarsTransfer Habitat remains in Mars orbit, unoccupied. The life supportsystem would remain in an operationally ready state during this periodable to resume operation with high reliability for the return flightquickly.

In the past, solid adsorbents have been used for CO₂ removal. However,liquid absorbents have significant advantages over solid adsorbents. Theability to pump the absorbent from scrubber to stripper stages allowsfor continuous absorption and regeneration of the sorbent, which isgenerally more stable and reliable than alternating adsorbent bedsbetween absorption and regeneration, and eliminates the need for acomplicated valve network. Liquid may also be easily replenished orexchanged without disassembly.

Existing state-of-the-art CO₂ removal systems include the Carbon DioxideRemoval Assembly (CDRA) aboard the ISS, which relies on solid zeoliteadsorbents that experience a particulate dusting problem and is higherin size, weight and power when compared to estimates of a liquid system.Other CO₂ removal systems include amine-based systems like those used onsubmarines. These amines are prone to outgassing of dangerous andodorous products, air oxidation, thermal degradation, and can becorrosive.

As can be seen, there is a need for improved apparatus and methods toremove contaminants from a supply air in environments such as deep spacevehicles.

SUMMARY OF THE INVENTION

In one aspect of the present invention, an environmental control systemcomprises an air conditioning subsystem; a contaminant removal subsystemdownstream of the environment to be conditioned; wherein the contaminantremoval subsystem includes: a first gas-liquid contactor-separator; asecond gas-liquid contactor-separator; and a dehumidifier disposedeither upstream of the first gas-liquid contactor-separator ordownstream of the second gas-liquid contactor-separator.

In a further aspect of the present invention, a contaminant removalsubsystem comprises a first gas-liquid contactor-separator; a secondgas-liquid contactor-separator downstream of the first gas-liquidcontactor-separator; and a dehumidifier disposed either upstream of thefirst gas-liquid contactor-separator or downstream of the secondgas-liquid contactor-separator.

In another aspect of the present invention, a contaminant removalsubsystem comprises a first gas-liquid contactor-separator; a secondgas-liquid contactor-separator downstream of the first gas-liquidcontactor-separator; and a condenser downstream of the second gas-liquidcontactor-separator; wherein the condenser is configured to: dischargeliquid water for recovery and reuse; discharge a gaseous contaminant forrecovery and reuse.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an environmental control systemaccording to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a contaminant removal subsystemaccording to an embodiment of the present invention;

FIG. 3 is a schematic diagram of a contaminant removal subsystemaccording to another embodiment of the present invention;

FIG. 4 is a schematic diagram of a contaminant removal subsystemaccording to a further embodiment of the present invention;

FIG. 5 is a schematic diagram of a contaminant removal subsystemaccording to an additional embodiment of the present invention;

FIGS. 6A-6B are side views of a scrubber according to an embodiment ofthe present invention;

FIGS. 7A-7B are side views of a scrubber according to another embodimentof the present invention;

FIGS. 8A-8B are side views of a stripper according to an embodiment ofthe present invention;

FIGS. 9A-9B are side views of a stripper according to another embodimentof the present invention;

FIG. 10 is a schematic view of a scrubber according to yet anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

Various inventive features are described below that can each be usedindependently of one another or in combination with other features.However, any single inventive feature may not address any of theproblems discussed above or may only address one of the problemsdiscussed above. Further, one or more of the problems discussed abovemay not be fully addressed by any of the features described below.

Broadly, the present invention can be integrated into environments suchas spacecraft used in long-duration missions, specifically, spacestations and spacecraft and habitats used in and beyond low earth orbit,as the invention, particularly gas-liquid separation, operatesindependent of gravity. The present invention may also be used foraircraft or submarines, as examples, where its gravity independencewould help mitigate failures due to managing liquids under turbulence orplacing the vehicle in an inverted position.

The present invention may be part of an environmental control system.The present invention can provide a contaminant (e.g., CO₂) removalsubsystem that may interface with an upstream temperature and humiditycontrol device which interfaces with an environment for occupants, suchas a cabin. Downstream of this removal subsystem, water may be storedfor water processing, whereas a contaminant outlet may interface with acontaminant reduction subsystem. It will be understood by those skilledin the art that incorporation of a contaminant removal subsystem into anintegrated environmental control system will be desirable in someapplications, including aircraft cabins, but for other applications, thesubsystem may be operated as an independent unit, or integrated withother subsystems which convert carbon dioxide into oxygen and otherbyproducts.

Air that will be processed using the system described in this patent maycome from occupied environments, and may contain substantial humidity.It is an important object of this invention that it removes carbondioxide from air containing humidity and produces a carbon dioxideproduct stream that is substantially free of contaminating water. In theevent that the process is intended to provide carbon dioxide to adownstream Sabatier reactor, this reactor can tolerate a waterconcentration in the carbon dioxide no greater than 2%. It is thereforean object of the invention to provide carbon dioxide that meets thispurity requirement.

This invention generally provides a metabolic CO₂ removal system forspacecraft which can include a continuous, liquid-based architecturehaving paired scrubbing and stripping hollow fiber membrane gas-liquidcontactors between which the ionic liquid absorbent is pumped. The highsurface area of the hollow fiber membrane contactors enables a high masstransfer of CO₂, humidity, and contaminant gases into the ionic liquidusing a small system volume and weight. The membrane contactor canensure that liquid and gas do not need further separation, enabling thedevice to act in a gravity-independent way without the use of movingparts.

This invention can flow the liquid absorbent counter-current throughhollow-fiber membranes to achieve the required mass transfer without theneed of a powered spray system and coalescer. It can also allow contactbetween the liquid absorbent and gaseous contaminants, without themixing of the liquid and air, which in other contactors would result inthe need for centrifugal separator machinery to separate the phases. Notmixing the air and liquid can also decrease the risk of contamination ofabsorbent into the cabin.

The same membrane-based device can be used as a stripper component toeliminate the need for a spray scrubber contactor. Whereas a spraystripper might experience ionic liquid buildup due to the lack ofgravity and lack of large movement of fluid through the chamber, amembrane contactor stripper can allow for easy fluid flow without theneed to generate centrifugal forces.

In an exemplary embodiment, the invention is in a regenerable cabin airCO2 and water control system of a full regenerable air-revitalizationsystem for spacecraft and distant habitats with long duration missions.Upstream of the CO2 removal device in this regenerable airrevitalization system might be a temperature and humidity controldevice. Downstream of the CO2 removal device might be a device whichprocesses the captured CO2 into water (and methane as a byproduct), orconverts it directly into oxygen (and carbon or carbon monoxide as abyproduct).

A membrane contactor could comprise a cylindrical module filled withparallel hollow porous fibers. Dimensions of these hollow fibers couldbe <3 mm, and the pore dimension could be <2 microns. Optionally,baffles or other structures may also be present between the fibers orbetween the fibers and the outer shell to improve mixing of the fluid onthe shell side. Also optionally, flow on the shell side may be swirledor turbulated using duct bends prior to entry into the membrane moduleand/or angling flow entry vector into the module and/or using guidevanes or similar structures to enhance mass transfer across themembrane. Ports on the two ends of the modules may connect to a manifold(typically called a tube sheet) allowing fluid flow from the portsthrough the bore of each fiber and hence to the opposing port. Twoadditional ports may access the shell-side at opposite ends of themodule, allowing fluid flow on the outside of the fibers through themodule. The material of the hollow fibers can be selected such that theionic liquid does not wet the pores, and the trans-membrane pressure iskept low enough to prevent pore penetration. Possible choices for thefiber material include hydrophobic materials such as polypropylene,polyvinylidene fluoride, polysulfone, polyimide andpolytetrafluoroethylene. Optionally, a coating can be applied either ofPTFE or a crosslinked siloxane, to prevent liquid flow through thepores. The ionic liquid flow can be either on the “tube” side or the“shell” side. Air is flowed on the other side.

In operation as a scrubber, clean ionic liquid could be flowed on oneside of the membrane, and air containing CO2 on the opposite side. Sincethe membrane is hydrophobic, vapor, including CO2 and water, could beallowed to cross the membrane fibers and be absorbed by the ionicliquid, and carried away to a reservoir, while the ionic liquid cannotpass through the membrane fibers, and are thus contained from theairstream.

In operation as a stripper, loaded ionic liquid could be flowed on oneside of the membrane, and a small flow of sweep gas on the oppositeside. The temperature of the liquid could be raised and the partialpressure of CO2 and water on the gas phase side could be decreased tocreate a driving force for CO2 and water to transfer. One way toaccomplish this would be to apply a vacuum on the gas side. The strippermay also take advantage of any differences in desorption temperature orpressure between CO2 and water to separate the two constituents withoutan additional separator device.

US patent application entitled “Apparatus and Methods for EnhancingGas-Liquid Contact/Separation” filed Feb. 1, 2017, Ser. No. 15/422,170;US patent application entitled “Ionic Liquid CO2 Scrubber forSpacecraft” filed 02-01-2017, Ser. No. 15/422,166; and US patentapplication entitled “Dual Stripper with Water Sweep Gas” filedconcurrently herewith, are incorporated herein by reference as thoughfully set forth herein.

Herein, the term “absorbent” is intended to generally include absorbentsand/or adsorbents.

FIG. 1 is a schematic diagram of an exemplary embodiment of anenvironmental control system (ECS) 100 according to the presentinvention. The ECS 100 may receive an outside air 101 which, forexample, may be a bleed air when the ECS 100 is implemented for anaircraft. The outside air 101 may flow to one or more sensors 102 thatmay sense characteristics of the outside air 101, such as temperatureand/or humidity.

From the one or more sensors 102, the outside air 101 may flow to one ormore sensors 115 that may sense characteristics of the air 101 such astemperature, pressure and/or humidity. Once past the sensor(s) 115, theoutside air 101 may be conditioned by an air conditioning subsystem 103,which conditioning may include temperature, pressure and/or humidity.

Conditioned air may flow from the air conditioning subsystem 103,through a mix manifold 104, and into an environment 105 to beconditioned. The environment 105 may be an enclosed area for occupants,such as a cabin of an aircraft. In the environment 105, the conditionedair may acquire contaminants, such as from occupants in the environment105, and produce a contaminated air.

Upon exiting the environment 105, the contaminated air may be sensed byone or more gas contaminant sensors 106. The sensor(s) 106 may sense oneor more gas contaminants, such as CO2. Once past the sensor(s) 106, thecontaminated air may be filtered by a filter 108. A part of filtered airfrom the filter 108 may then be sensed by one or more VOC sensor(s) 107that may sense one or more volatile organic compounds (VOCs). After thesensor(s) 107, the part of a filtered air from the filter 108 may moveinto a recirculation fan 110. From the recirculation fan 110, thefiltered air may be forced into the mix manifold 104 where the filteredair can be mixed with the conditioned air from the air conditioningsubsystem 103.

Instead of and/or in addition to the filtered air moving through the fan110, a part of filtered air from the filter 108 may move through acontaminant removal subsystem 111. The removal subsystem 111 may removeone or more gas contaminants sensed by the one or more sensors 106. Thecontaminant removal subsystem 111 may, in various embodiments, includeone or more gas-liquid contactor and separators, such as those describedbelow, to effectuate removal of gas contaminants. However, the presentinvention envisions that gas-liquid contactor and separators, other thanthose described below, can be employed.

Within the contaminant removal subsystem 111, a used liquid absorbentmay be produced. “Used liquid absorbent” means “clean liquid absorbent”that has absorbed gas contaminant(s). “Clean liquid absorbent” meansliquid absorbent that is substantially free of absorbed gascontaminant(s).

Also within the contaminant removal subsystem 111, a regenerated liquidabsorbent may be produced. “Regenerated liquid absorbent” means usedliquid absorbent that has undergone desorption of gas contaminant(s).

Additionally within the contaminant removal subsystem 111, a cleaned airmay be produced. “Cleaned air” means air that has an insubstantialamount of gas contaminant(s) and/or H2O. In embodiments, “cleaned air”has a gas contaminant(s) and/or H2O concentration less than that of theoutside air 101 and/or less than that of the contaminated air from theenvironment 105.

Cleaned air from the subsystem 111 may flow into a photocatalyticoxidizer (PCO) 112 to remove one or more VOCs and thereby produce afurther cleaned air. One or more VOC sensors 113 may be downstream ofthe PCO 112. One or more gas contaminant sensors 109, such as CO2sensor(s), can be downstream of the VOC sensor(s) 113. The furthercleaned air flow can then flow into the fan 110, and then into a mixmanifold 104 where it can be mixed with conditioned air from the airconditioning subsystem 103.

Instead of and/or in addition to flowing through the PCO 112, a part ofcleaned air (e.g., <10%) from the contaminant removal subsystem 111 mayrecirculate back into the subsystem 111 for additional processing suchas desorption of gas contaminant(s) from used liquid absorbent, andregeneration of clean liquid absorbent, as further described below.

A controller 114 may be in communication with one or more of the sensors106, 109, 113, 115 for control over one or more components of the ECS100, such as fan(s) and/or and valve(s) (not all of which may be shownin FIG. 1).

FIG. 2 is a schematic diagram of an exemplary contaminant removalsubsystem (i.e., closed-loop air revitalization subsystem) 200A that maybe employed in the aircraft ECS 100 above and/or in a space-basedsystem. However, other contaminant removal subsystems may be employed. A“closed-loop air revitalization subsystem” is intended to mean asubsystem which recovers valuable resources from waste products, such asrecovering valuable oxygen from waste carbon dioxide. The subsystem 200Amay include one or more gas-liquid contactor-separators to effectuateremoval of gas contaminant(s), such as those described below. However,the subsystem 200A is not limited to the contactor-separators describedbelow.

The contaminant removal subsystem 200 may receive a contaminated air 201from an environment, such as a spacecraft cabin. The contaminated air201 may include one or more gas contaminants such as CO2, and/or H2O,and the air 201 may flow into a first gas-liquid contactor-separator(i.e., scrubber) 202. In embodiments, the contaminated air 201 may,before entering the scrubber 202, be filtered for dust and particulates,via a filter 210, as well as being forced, via a fan 211, into thescrubber 202.

Concurrent with, or sequentially with, the scrubber 202 receiving thecontaminated air 201, a clean liquid absorbent may be pumped, via a pump212, into the scrubber 202, from a clean liquid absorbent storage 205.In embodiments, the liquid absorbent may be one or more ionic liquidsdescribed below.

Before entering the scrubber 202, the clean liquid absorbent may becooled by a cooler 213 disposed between the pump 212 and the scrubber202.

From the scrubber-separator 202 cleaned air 203 may optionally flowthrough a filter 214, to capture any leaked ionic liquid and/or producea further cleaned air that can flow back to the environment to beconditioned. In embodiments, the cleaned air 203 may have a gascontaminant(s) concentration, and/or H2O concentration lower than thatof the contaminated air 201.

Also from the scrubber-separator 202 used liquid absorbent may exit. Theused liquid absorbent may flow into a heat exchanger 206. Therein, theused liquid absorbent may be heated by a regenerated liquid absorbentdescribed below, and next flow, via a pump 221, into a heater 207wherein the used liquid absorbent may be further heated. Alternatively,in the context of an aircraft, the used liquid absorbent may be heatedby trim air.

From the heater 207, a heated, used liquid absorbent (i.e., absorbentliquid with contaminants) may be received by a second gas-liquidcontactor-separator 208 (i.e., stripper) 208. The stripper 208 may havethe same design as the scrubber 202, or a different design. In thecontext of an aircraft, the stripper 208 may also be used to discharge(i.e., not recirculate) carbon dioxide and/or water which can betransferred to the trim air.

From the stripper 208, as noted above, a regenerated or clean liquidabsorbent may exit. In embodiments, a gas contaminant concentration inthe regenerated liquid absorbent is lower than that of the used liquidabsorbent. The regenerated liquid absorbent may flow into the heatexchanger 206 wherein the regenerated liquid absorbent may be cooled byused liquid absorbent.

Concurrent with the stripper 208 discharging or outflowing theregenerated or clean liquid absorbent, the stripper 208 may discharge oroutflow contaminant(s), such as CO2, and/or H2O vapor. As will becomemore evident below, the stripper 208 can be configured to not onlydischarge contaminants, but do so in a fashion that allows contaminantsto be recovered and reused.

A vacuum pump and/or compressor 217 may pump the dischargedcontaminant(s) from the stripper 208 and into a condenser 218. Thecondenser 218 can be configured to allow contaminants to be recoveredand reused. Therefore, in embodiments, the condenser 218 may dischargeliquid water, via a water separator 222, to a water storage 216 and/ordischarge CO2 and/or water vapor to a dehumidifier 219.

The dehumidifier 219 can be configured to enable the recovery and reuseof contaminants. Thus, the dehumidifier 219 may separate incoming watervapor from CO2, and flow the CO2 to a Sabatier reactor 215. Thedehumidifier 219 can also receive, via a needle valve 220, a mixture ofCO2 and/or water vapor from the water separator 222, and use suchmixture as a sweep gas in the dehumidifier 219. The sweep gas mixture,together with water vapor that has been separated from CO2 in thedehumidifier 219, may then exit the dehumidifier 219 and flow back tothe stripper 208 as a sweep gas therein.

As can be seen, in embodiments, the CO2 and/or H2O may be recovered andreused, such as by the Sabatier reactor 215 and by the water storage216, respectively. A Sabatier reactor functions by reacting the carbondioxide with hydrogen to convert it to methane and water. The water, inturn, may be electrolyzed to generate hydrogen and oxygen, forming aclosed-loop air revitalization system.

FIG. 3 is a schematic diagram of another exemplary contaminant removalsubsystem (i.e., closed-loop air revitalization subsystem) 300A that issimilar to the subsystem 200A shown in FIG. 2. Accordingly, referencenumbers in FIG. 3 correspond to those in FIG. 2.

However, in the embodiment of FIG. 3, a sweep gas to a stripper 308 isprovided by a separate water vaporizer 323 rather than a dehumidifier.More specifically, and unlike the embodiment of FIG. 2, in theembodiment of FIG. 3, the water vaporizer 323 is intermediate a waterseparator 322 and a water storage 316. Accordingly, the water separator322 can outflow liquid water to the water vaporizer 323. In turn, thewater vaporizer 323 can outflow water vapor to the stripper 308 whereinthe water vapor can act as a sweep gas.

In addition, in the embodiment of FIG. 3, the dehumidifier 319, uponreceiving, via a needle valve 320, a mixture of CO2 and/or H2O vapor,may use such mixture therein as a sweep gas. Upon such sweep gas mixtureexiting the dehumidifier 319, the sweep gas mixture may recirculate backto a point upstream of (i.e., at the suction side of) the compressor317.

FIG. 4 is a schematic diagram of an additional exemplary contaminantremoval subsystem (i.e., closed-loop air revitalization subsystem) 400Athat is similar to the subsystem 200A shown in FIG. 2. Accordingly,reference numbers in FIG. 4 correspond to those in FIG. 2.

However, in the embodiment of FIG. 4, a dehumidifier 420 is intermediatea source of contaminated air, such as an occupant cabin, and a scrubber402, rather than the dehumidifier being downstream of the stripper as inthe embodiment of FIG. 2.

Accordingly, the dehumidifier 420 may receive contaminated aircontaining CO2 and humidity from the blower 411, and remove water fromthis air. The dehumidified air passes to the scrubber 402. In thescrubber, CO2 and more water is removed. The now very dry and purifiedair is returned to the dehumidifier 420 as a sweep gas. As a sweep gas,it receives the water removed from the other stream, and isrehumidified. This stream is now returned to the cabin as clean cabinair. In this embodiment, there is no sweep stream provided to thestripper 408.

FIG. 5 is a schematic diagram of a still further exemplary contaminantremoval subsystem (i.e., closed-loop air revitalization subsystem) 500Athat is similar to the combined subsystems 300A and 400A shown in FIGS.3-4. Accordingly, reference numbers in FIG. 5 correspond to those inFIGS. 3-4.

In the embodiment of FIG. 5, like the embodiment in FIG. 3, a sweep gasto a stripper 508 is provided by a separate water vaporizer 523 ratherthan a dehumidifier. In the embodiment of FIG. 5, the water vaporizer523 is intermediate a water separator 522 and a water storage 516.Accordingly, the water separator 522 can outflow liquid water to thewater vaporizer 523. In turn, the water vaporizer 523 can outflow watervapor to the stripper 508 wherein the water vapor can act as a sweepgas.

Also, in the embodiment of FIG. 5, like the embodiment in FIG. 4, adehumidifier 520 is intermediate a source of contaminated air, such asan occupant cabin, and a scrubber 502, rather than the dehumidifierbeing downstream of the stripper as in the embodiment of FIG. 2.

Accordingly, the dehumidifier 520 may receive contaminated aircontaining CO2 and humidity from the blower 511, and remove water fromthis air. The dehumidified air passes to the scrubber 502. In thescrubber, CO2 and more water is removed. The now very dry and purifiedair is returned to the dehumidifier 520 as a sweep gas. As a sweep gas,it receives the water removed from the other stream, and isrehumidified. This stream is now returned to the cabin as clean cabinair. In this embodiment, a sweep stream, in the form of H20 vapor from awater vaporizer 523, is provided to the stripper 508.

FIGS. 6A-6B depict an exemplary embodiment of a scrubber 602 that may beemployed in the contaminant removal subsystem 200, for example. In FIG.6A, the scrubber 602 may include a cylindrical housing 602 a thatencloses a hollow fiber bundle 602 b. Contaminated air may enter thehousing 602 a at one end thereof and clean air may exit at an oppositeend thereof. Regenerated or clean absorbent liquid may enter the housing602 a at one side thereof, and used liquid absorbent with contaminantsmay exit the housing 602 a at an opposite side thereof. In thisembodiment, regenerated or clean absorbent liquid flows counter (i.e.,opposite) to the contaminated air flow. Moreover, the counter flowcauses contaminants to flow radially outward from the hollow fiberbundle 602 b.

FIG. 6B depicts the same flows as in FIG. 6A, but in the context of asingle hollow fiber 602 c that can be part of the hollow fiber bundle602 b.

FIGS. 7A-7B depict another exemplary embodiment of a scrubber 702 thatmay be employed in the contaminant removal subsystem 200, for example.As in FIG. 6A, in FIG. 7A, the scrubber 702 may include a cylindricalhousing 702 a that encloses a hollow fiber bundle 702 b. However,contaminated air may enter the housing 702 a at one side thereof andclean air may exit at an opposite side thereof. Regenerated or cleanabsorbent liquid may enter the housing 702 a at one end thereof, andused absorbent liquid with contaminants may exit the housing 702 a at anopposite end thereof. As in FIG. 6A, in this embodiment, clean absorbentliquid flows counter (i.e., opposite) to the contaminated air flow.However, the counter flow causes contaminants to flow radially inwardtowards the hollow fiber bundle 702 b.

FIG. 7B depicts the same flows as in FIG. 7A, but in the context of asingle hollow fiber 702 c that can be part of the hollow fiber bundle702 b.

FIGS. 8A-8B depict an exemplary embodiment of a stripper 808 that may beemployed in the contaminant removal subsystem 200, for example. In FIG.8A, the stripper 808 may include a cylindrical housing 802 a thatencloses a hollow fiber bundle 802 b. Sweep gas may enter the housing802 a at one end thereof and contaminants may exit at an opposite endthereof. Used absorbent liquid with contaminants may enter the housing802 a at one side thereof, and regenerated or clean absorbent liquid mayexit the housing 802 a at an opposite side thereof. In this embodiment,used absorbent liquid with contaminants flows counter (i.e., opposite)to the sweep gas flow. Moreover, the counter flow causes contaminants toflow radially inward towards the hollow fiber bundle 802 b.

FIG. 8B depicts the same flows as in FIG. 8A, but in the context of asingle hollow fiber 802 c that can be part of the hollow fiber bundle802 b.

FIGS. 9A-9B depict another exemplary embodiment of a stripper 908 thatmay be employed in the contaminant removal subsystem 200, for example.As in FIG. 8A, in FIG. 9A, the stripper 908 may include a cylindricalhousing 902 a that encloses a hollow fiber bundle 902 b. However, sweepgas may enter the housing 902 a at one side thereof and contaminants mayexit at an opposite side thereof. Used absorbent liquid withcontaminants may enter the housing 902 a at one end thereof, andregenerated or clean absorbent liquid may exit the housing 902 a at anopposite end thereof. As in FIG. 8A, in this embodiment, sweep gas flowscounter (i.e., opposite) to the used absorbent liquid with contaminantsflow. However, the counter flow causes contaminants to flow radiallyoutward from the hollow fiber bundle 902 b.

FIG. 9B depicts the same flows as in FIG. 9A, but in the context of asingle hollow fiber 902 c that can be part of the hollow fiber bundle902 b.

FIG. 10 depicts yet another exemplary embodiment of a scrubber 1002 thatmay be employed in the contaminant removal subsystem 200, for example.As a further example, the scrubber 1002 may be employed as the scrubber202 and/or 302. In FIG. 10, the scrubber 1002 may include a plurality ofparallel housings 1002 a, b, c that enclose respective hollow fiberbundles (not shown) that can be similar to those in FIGS. 6A-6B or FIGS.7A-7B.

Although FIGS. 6-10 describe a hollow fiber and/or bundle in the contextof a stripper and/or scrubber, it should be understood that the same maybe employed in the context of a dehumidifier, such as that described inC. lacomini, J. Hecht, J. Harrell, J. Lumpkin “Qualification of theBoeing Starliner Humidity Control Subassembly”, 46th InternationalConference on Environmental Systems, 10-14 Jul. 2016, Vienna, Austria,ICES-2016-322, which is incorporated herein by reference.

According to the present invention, the liquid absorbent can meet ademanding set of criteria. The liquid can be safe and nontoxic tohumans, and may not contaminate the purified air with odors or organicvapors. It may absorb carbon dioxide at the partial pressure expectedduring the mission, and may not lose performance when simultaneouslyabsorbing water. It may also be regenerable without the use of spacevacuum, so as not to lose CO2 and water to space, and regenerablewithout using excessive temperatures or power. The liquid may be durableand last without deterioration for the life of the mission.

The liquid absorbent can be one or more ionic liquids. They are salts,generally comprised of an anion and organic cation, which are liquid attheir temperature of use. Because they are salts, they have effectivelyzero vapor pressure, thus eliminating odors and reducing the likelihoodof contaminating the purified air. They are generally nontoxic and havesufficient stability to resist deterioration. Ionic liquids generallycontain relatively large organic cations (quaternary ammonium orphosphonium compounds) and any of a variety of anions, both of which canbe tailored to obtain desired characteristics. Ionic liquids can bothphysically dissolve carbon dioxide and have specific chemicalinteractions with it. As a class, almost every ionic liquid is watersoluble and hygroscopic, meaning that they will absorb moisture from theair, but due to their negligible volatility, the water can be removed byevaporation either by elevating the temperature or reducing the waterpartial pressure. Because a very large number of ionic liquids exist,and both the cation and the anion can be tailored to obtain desiredcharacteristics, this class of compounds has flexibility as the liquidabsorbent for a carbon dioxide removal system with ability to removecontaminants.

Ionic liquids suitable for use in this invention comprise those withmelting points below 20° C., low vapor pressure, and with capacity forcarbon dioxide, at 30 deg. C and in the presence of 3.8 torr carbondioxide partial pressure, of >0.3%. Examples of such ionic liquidsinclude 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazoliumtrifluoracetate, 1-butyl-3-methylimidazolium acetate,tributylmethylphosphonium acetate, triethylmethylphosphonium acetateetc. These ionic liquids are hygroscopic and can absorb water as welland carbon dioxide. Therefore, the effective working fluid can, in manycases, comprise a mixture of the ionic liquids specified and water. Itmay, under some circumstances, be useful to add water to the ionicliquid before contacting with carbon dioxide. This can reduce the carbondioxide capacity but also reduce the viscosity.

1-butyl-3-methylimidazolium acetate (BMIM Ac) has a high CO2 capacityand well understood physical properties. BMIM Ac satisfies the basicrequirements for an absorbent in a manned vehicle. It is not a hazardoussubstance or mixture, and has no hazards not otherwise classified. ThepH of an aqueous solution is 6.1, and the autoignition temperature is435° C. The compound is a clear, somewhat viscous liquid, and can behandled readily. The surface tension is similar to that for a polarorganic solvent, and the density is similar to that for water. The onsetfor thermal degradation sets the upper temperature limit for processing,and is comfortably higher than the temperature needed for desorption.The viscosity for this ionic liquid is higher than that of water, butcan be reduced either by raising the temperature or water content. Innormal use, the ionic liquid absorbs both CO2 and water, and thereforethe viscosity values vary in the presence of water. Viscosity plays arole in determining mass transfer rates for CO2 adsorption anddesorption. Control of viscosity can therefore reduce the weight andvolume of the contactor-separator.

EXAMPLES

For mass transfer in membrane systems, we should consider transport inthe gas phase, transport through the membrane, and liquid-phase masstransport. Gas-phase transport will be fast relative to the otherprocesses, and it is possible to minimize the resistance to masstransfer through the membrane by proper material and morphology choices,leaving liquid phase mass transfer as the rate-determining process. Toprevent the membrane from significantly slowing mass transfer, anon-wetting membrane material should be chosen, since liquid-filledpores create stagnant zones that inhibit liquid flow. For liquid-phasemass transfer, the mass transfer coefficient is expected to be stronglydependent on the diffusion coefficient, and this, in turn, dependsprimarily on the ionic liquid viscosity. The ratio between the masstransfer coefficient and the diffusion coefficient is determined by theSherwood number. The Lévèque-Graetz and Kartohardjono approaches toestimating this number include dependencies on the velocity of flowthrough the fiber, liquid viscosity and the diffusion coefficient forCO2 in the liquid.

${Sh} = \sqrt[3]{3.67^{3} + {1.62^{3}\frac{{vd}^{2}}{DZ}}}$${Sh} = {0.1789{\varphi^{0.86}( \frac{\rho\;{vd}}{\eta} )}^{0.34}( \frac{\eta}{\rho\; D} )^{\frac{1}{3}}}$

Avoiding wetting the membrane pores not only improves mass transfer butalso prevents leakage of ionic liquid into gas lines. Such leakage wouldnecessitate subsequent separation. The ability of the liquid topenetrate pores depends on the surface tension, the viscosity, thedimension of the pores and the contact angle. Because ionic liquids arepolar and BMIM Ac has a surface tension of 36.4 mN m−1, theseconsiderations guide us to investigate relatively hydrophobic membranematerials with low critical surface tensions, such aspolytetrafluoroethylene (PTFE). We measured the contact angle for BMIMAc on a porous PTFE surface to be 81.3, showing that it will not wetthis material unless significant force is applied. Kreulen H, Kreulen,C. A. Smolders, G. F. Versteeg, W. P. M. van Swaaij “Microporous hollowfiber membrane modules as gas liquid contactors. Part 1. Physical masstransfer processes, A specific example: Mass transfer in highly viscousliquids” J. Membrane Sci., vol. 78, 1993, 197-216 and Z. Dai, L.Ansaloni, L. Deng “Precombustion CO2 capture in polymeric hollow fibermembrane contactors using ionic liquids: Porous membrane versusnonporous composite membrane” Ind. Eng. Chem., Res. Vol. 55, 2016,5983-5992 each describe the addition of a composite layer to preventpore-filling with very little effect on mass transfer.

Initial experimental results using a membrane contactor were obtainedusing a laboratory test stand. A hollow fiber microfiltration module wasused for the contactor, with 90:10 BMIM Ac:water as the liquid phase,and air containing 1-4 torr partial pressure of CO2 at atmosphericpressure as the vapor phase. The pressures of both the liquid and thegas phases were controlled at up to 6 psi in operation. The ionic liquidmay either be directed through the lumina of the hollow fibers orthrough the shell surrounding them, and we evaluated both options. Whenthe ionic liquid passes through the lumina, the pressure drop is higherbecause of the viscosity of the ionic liquid, and there is lessopportunity for bypass due to the small diameter of these fibers. Infact, we observed little difference between results from these twoconfigurations. Equation (8) defines the mass transfer coefficient asthe ratio of the molar flux to driving force (either concentrationdifference or partial pressure difference), and has been used toestimate the membrane area required for a CO2 load of 4.15 kg day-1,representing a likely load from four crew members in a deep spacevessel.

$k = {\frac{n_{{CO}\; 2}}{A\;\Delta\; c_{{CO}\; 2}} = \frac{n_{{CO}\; 2}{RT}}{A\;\Delta\; p_{{CO}\; 2}}}$

CO2 diffusivity in the ionic liquid is expected to be a main parameterthat defines the overall mass transfer and the process efficiency. Thediffusivity of CO2 in air is very high in comparison with diffusion inthe ionic liquid, so the main resistance to CO2 mass transfer is in theliquid phase. Addition of small amounts of water into the liquiddecrease the viscosity and increase the diffusivity. We have shown thatthe water content in ionic liquid negatively affects the absorptioncapacity. However, the effect of water dilution on capacity is expectedto be smaller than the positive effect of water dilution on liquid phasemass transfer rate via viscosity reduction, and by increasing theair-liquid surface area. In other words, the dilution of the ionicliquids with water that is inevitable because of the affinity of theionic liquids for water is expected to increase the mass transfer rate(decrease scrubber size) without greatly decreasing the ionic liquidcapacity (i.e. increasing the flow rate of ionic liquid).

Similarly, we have shown that this process relies on exposing a largesurface area of ionic liquid to the air stream. In addition to masstransfer, heat transfer will also occur. Note that the ionic liquidentering the scrubber is cooled. This is beneficial because it increasesthe working CO2 loading capacity, but will also have the effect ofcooling the air returning to the cabin. Again, by taking advantage ofthis process, the load on cooling systems elsewhere in the spacecraft isreduced, resulting in weight and power reductions for these systems.

It is estimated that this system will be a low-maintenance,high-reliability system since the ionic liquid system will not havecorrosion problems.

In addition to reducing the launch costs associated with the use of theionic liquid system, the new system would also reduce the mass ofconsumables. The direct liquid contact system would not vent to spaceand would not have a connection to space vacuum. The use of theopen-loop CO2 removal systems typically used in short-duration missionshas also been considered for long-duration missions due to theirsimplicity and low size, weight and power. However, these devices wouldrequire an even greater launch cost due to consumables. Anderson, M. A.,Ewert, M. K., Keener, J. F., Wagner, S. A., Stambaugh, I. C. NASA ReportTP-2015-21870, Johnson Space Center, March, 2015 estimated that the useof an open-loop system would require a total of 21 kg/crewmember/day ofpotable water, oxygen, and the tanks to store them, or a total of about75,600 kg for a 30 month mission to Mars with 4 crewmembers.

The combination of direct liquid contact and novel ionic liquids canprovide an integrated carbon dioxide, humidity and trace contaminantremoval system with a significantly lower size, weight and powercompared to current systems. This system gains its advantage from theuse of ionic liquid as the liquid absorbent. Liquid systems eliminatethe mass transfer limitations and plumbing complexities of alternativesolid adsorbents, and avoid contamination of the cabin air by thesorbent through the use of ionic liquids, which have zero vaporpressure. In addition to being containable, ionic liquids are flameretardant, non-toxic, and very stable. This sorbent is completelyregenerable, without the use of space vacuum or high temperatures, andthus provides NASA with an excellent option for closed-loop CO2 removalon existing and future vehicles.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

We claim:
 1. An environmental control system, comprising: an airconditioning subsystem; a contaminant removal subsystem downstream ofthe environment to be conditioned; wherein the contaminant removalsubsystem includes: a first gas-liquid contactor-separator; a secondgas-liquid contactor-separator; and a dehumidifier disposed upstream ofthe first gas-liquid contactor-separator to produce a sweep gas to thefirst gas-liquid contactor-separator.
 2. The contaminant removalsubsystem of claim 1, further comprising a condenser downstream of thesecond gas-liquid contactor separator.
 3. The system of claim 1, whereinthe first and second gas-liquid contactor-separators comprise a hollowfiber membrane bundle.
 4. The contaminant removal subsystem of claim 1,further comprising a vacuum pump downstream of the second gas-liquidcontactor-separator.
 5. A contaminant removal subsystem, comprising: afirst gas-liquid contactor-separator; a second gas-liquidcontactor-separator downstream of the first gas-liquidcontactor-separator; and a dehumidifier disposed upstream of the firstgas-liquid contactor-separator to produce a sweep gas to the firstgas-liquid contactor-separator.
 6. The subsystem of claim 5, furthercomprising a condenser downstream of the second gas-liquidcontactor-separator.
 7. The subsystem of claim 5, further comprising acompressor downstream of the second gas-liquid contactor-separator. 8.The subsystem of claim 5, wherein the liquid in the gas-liquidcontactor-separator is an ionic liquid.
 9. The subsystem of claim 5,wherein the first and the second gas-liquid contactor-separatorscomprise hollow fiber membrane bundles.
 10. A contaminant removalsubsystem, comprising: a first gas-liquid contactor-separator; a secondgas-liquid contactor-separator downstream of the first gas-liquidcontactor-separator; a dehumidifier upstream of the first gas-liquidcontactor-separator and configured to produce a sweep gas to the secondfirst gas-liquid contactor-separator; and a condenser downstream of thegas-liquid contactor-separator; wherein the condenser is configured to:discharge liquid water for recovery and reuse; discharge a contaminantfor recovery and reuse.
 11. The subsystem of claim 10, furthercomprising a vacuum pump downstream of the second gas-liquidcontactor-separator.
 12. The subsystem of claim 10, wherein thecondenser is further configured to discharge contaminants that includeCO2.
 13. The subsystem of claim 10, wherein the condenser is furtherconfigured to discharge contaminants that include water.
 14. Thesubsystem of claim 10, further comprising a water extractor downstreamof the condenser.
 15. The subsystem of claim 10, wherein the dischargedcontaminant is CO2.